(1) Field of the Invention
The present invention relates to a process for the preparation of a 3,3', 4,4'-biphenyltetracarboxylic acid salt by dehalogenodimerization of a 4-halogenoorthophthalic acid salt.
(2) Description of the Prior Art
As the conventional process for the preparation of 3,3', 4,4'-biphenyltetracarboxylic acid, there is known a process in which an aqueous solution in which a 4-halogenoorthophthalic acid salt and an alkali metal hydroxide are dissolved is heated at 50 to 150.degree. C. in the presence of a catalyst having metallic palladium supported on a carrier and a small amount of methanol and the reaction is conducted for 0.2 to 30 hours (see Japanese Patent Application Laid-Open Specification No. 20705/80). In this dehalogenodimerization reaction, however, the yield is not high. That is, the yield is about 29% to about 58.6%, 3,3', 4,4'-Biphenyltetracarboxylic dianhydride prepared from this 3,3', 4,4'-biphenyltetracarboxylic acid salt can be used as a starting material of a polyimide or an epoxy-curing agent. From the industrial viewpoint, in order for this salt to be used as the starting material of various polymers, it is desirable that the salt be obtained in a high yield, for example, a yield of at least 80%. If the yield is low, the cost of the starting material of a polyimide which is a heat-resistant resin is increased.